When microelectronic devices are produced on a semiconductor wafer, it is crucial that the critical dimensions of the devices be held within specified tolerances. Critical dimensions, in this context, refer to the widths of features, such as conductors, that are deposited on the wafer and the spacing between adjacent features. Deviations from the specified dimensions lead to performance and yield degradation. The manufacturing process must therefore be carefully monitored and controlled, in order to detect deviations as soon as they occur and to take corrective action to avoid the loss of costly wafers in process. For example, when a critical dimension in photoresist that has been deposited and etched on the wafer is found to be out of specification, it is possible to remove and reapply it.
A variety of systems and methods for measurement of critical dimensions are known in the art. Most microelectronic production facilities currently use optical metrology to monitor critical dimensions. As semiconductor devices become ever denser, however, with design rules of 0.25 μm and below, it becomes impossible For classical optical metrology systems to provide accurate results. Electron beam (e-beam) metrology has been suggested as an alternative, but e-beam systems also suffer from performance limitations.
X-ray measurement of critical dimensions is described in U.S. Pat. No. 6,556,652, whose disclosure is incorporated herein by reference. According to the method described in this patent, a surface of a substrate is irradiated with a beam of x-rays. A pattern of the X-rays scattered from the surface due to features formed on the surface is then detected and analyzed to measure a dimension of the features in a direction parallel to the surface.
Additional work on X-ray based CD measurements is described by Jones et al., in “Small Angle X-ray Scattering for Sub-100 nm Pattern Characterization,” Applied Physics Letters 83:19 (2003), pages 4059-4061, which is incorporated herein by reference. The authors use transmission-mode small angle X-ray. scattering (SAXS) with a synchrotron X-ray source to characterize a series of polymer photoresist gratings formed on a substrate. The X-ray beam passes through the grating and the substrate, and the SAXS pattern is measured using a two-dimensional CCD detector. The photoresist grating produces a one-dimensional series of diffraction spots in the SAXS pattern on the detector. The SAXS intensity as a function of the scattering vector q is analyzed to determine the grating spacing and sidewall angle. (According to convention in the X-ray scattering art, q =4π sin θ/λ, wherein 2θ is the scattering angle relative to the Incident beam, and λ is the X-ray wavelength.)